* Image is for illustrative purposes only.Mizuki (Water Fortress)
水城（みずき）
Mizuki (Water Fortress) is a historic castle in Kyūshū dating from the Built in 664 AD (3rd year of Emperor Tenji), whose lord was Yamato Court.
Overview
Background of Construction
Mizuki (Water Fortress) is an ancient defensive installation constructed in 664 AD (3rd year of Emperor Tenji) by the Yamato Court in Chikuzen Province (present-day Dazaifu City, Onojo City, and Kasuga City, Fukuoka Prefecture). Although it is designated with the character for "castle" (jo), it is closer in structure to a linear defensive embankment (bōrui) than a conventional castle, and is recognized as Japan's oldest and largest linear defensive installation. The direct background for its construction was the overwhelming defeat of the Japan-Baekje allied forces at the Battle of Baekgang in 663 AD (2nd year of Emperor Tenji), making the urgent development of a defensive line protecting Dazaifu — the administrative center of Kyushu — against Tang and Silla invasion a matter of national priority.
Structure and Scale
Mizuki is an earthwork embankment constructed using the hangchiku tamped-earth technique (compacting soil in layers), measuring approximately 1.2 km in total length, approximately 80 m in base width, and approximately 9–13 m in height (figures vary by source), with a top width of approximately 25 m. A moat approximately 60 m wide and approximately 4 m deep was constructed on the north side (facing Hakata Bay), with multiple large wooden conduits (mokuhi) used to channel water. City gates (east gate and west gate) were placed at the eastern and western ends of the embankment, combining traffic and logistics control functions. This structure crossed the Fukuoka Plain from mountain ranges to the coast, functioning as a "defensive wall" blocking the advance toward Dazaifu.
Role in the Dazaifu Defensive Line
Mizuki formed part of an integrated defensive system in combination with Ono-jo Castle (mountain fortress on Mt. Shioujisan, 665 AD) and Kii Castle (Saga Prefecture, 665 AD). Mizuki was responsible for halting advances across the flatland, while Ono-jo and Kii Castle functioned as surveillance and refuge bases from the mountains — a three-dimensional defensive system. This defensive structure was established against the backdrop of international tension in 7th–8th century East Asia and constitutes important historical evidence of Japan's national sense of crisis at the time.
Records in the Nihon Shoki
Regarding Mizuki, the Nihon Shoki records under the 3rd year of Emperor Tenji (664 AD): "A great embankment was built in Chikushi Province to store water, which was named Mizuki." This brief record is the sole contemporary historical source conveying the name and construction date of Mizuki, making it a particularly notable installation in ancient Japanese military infrastructure.
Today
The Mizuki earthwork remains survive in good condition and were designated a National Special Historic Site in 1953 (Showa 28). In 2017, it was selected as No. 182 in the Continued 100 Famous Castles of Japan. Developed as a historic site spanning Dazaifu City, Onojo City, and Kasuga City, the Mizuki-kan museum in Onojo City features an exhibition of a cross-section of the earthworks. JR Suiki Station takes its name from this Mizuki and is a beloved local landmark. It was also designated a component cultural heritage of the 2020 Japanese Heritage "Ancient Japan's Western Capital — East Asian Exchange Hub."
Connection to Swords
The late 7th century when Mizuki was built was an era when ironworking techniques from the continent (Tang China and Baekje) were being transmitted to the Japanese archipelago through Kyushu as a gateway. Dazaifu and its defensive installations also served as hubs for the production and management of iron weapons and agricultural tools, and soldiers deployed around Mizuki are believed to have been equipped with continental-style swords. The Baekje refugee engineers who arrived after the defeat at the Battle of Baekgang were also specialists in smithing and metalworking, and are thought to have indirectly contributed to the development of Japanese sword-making techniques.
Highlights
- National Special Historic Site; Continued 100 Famous Castles No. 182
- Oldest linear defensive installation, built the year after the Battle of Baekgang (663 AD)
- Massive hangchiku earthwork: 1.2 km long, 80 m base width
- Sole contemporary historical source recorded in the Nihon Shoki
- 2020 Japanese Heritage 'Ancient Japan's Western Capital' component site
* Opening hours and admission fees are subject to change. Please check the official website before visiting.